1. Field of Invention
The present invention is in the field of electronics, and more particularly, is directed to radio frequency amplifiers.
2. Discussion of Related Art
Solid state Class-C power transistors used for pulsed radio frequency (RF) power amplifiers suffer from frequency spectrum problems, mainly caused by ringing from the pulse falling edge. Consequently, conventional high power, pulsed, multi-stage, RF power amplifiers that require frequency spectrum control use either active pulse/tail clipper circuitry, active analog pulse ramp control, or a passive cavity bandpass filter to provide the needed control.
Referring to FIG. 1, there is illustrated a block diagram of a three-stage power amplifier using pulse/tail clipper circuitry for frequency spectrum control. The three-stage power amplifier 100 includes three RF power transistors 110 coupled between an input 120 and output 130 of the amplifier. Impedance matching networks 140a and 140b are coupled between the input 120 and the first RF power transistor 110 and between the third RF power transistor and the output 130, respectively, as shown in FIG. 1. Impedance matching networks 140c and 140d are provided between the RF stages, as also shown in FIG. 1. The frequency control block 150 includes a pulse clipper drive circuit 160 coupled to the impedance matching network 140d between the second and third RF power transistors 110 (i.e., between the middle and end stages) via a capacitor 170.
Another conventional method to achieve spectral compliance includes using an active analog pulse ramp/fall time control system at the beginning of the multi-stage power amplifier chain. An example of a three-stage power amplifier 100 including ramp/fall time control circuitry 180 for frequency spectrum control is shown in FIG. 2. The ramp/fall time control circuitry 180 is coupled to the output of the first RF power transistor 110, i.e., to the beginning stage of the amplifier 100. FIG. 3 illustrates an example of the three-stage power amplifier 100 including circuitry for the third conventional method of frequency spectrum control, namely, using a passive cavity filter at the end of the multi-stage power amplifier chain. As shown in FIG. 3, a cavity bandpass filter 190 is coupled between the output of the impedance matching network 140b and the output 130 of the power amplifier 100.
Each of three conventional methods of frequency spectral control has associated disadvantages and drawbacks. The active pulse/tail clipper and pulse ramp/fall time control methods use complicated circuitry and require a large footprint on a printed circuit board design. In addition, the pulse clipper and pulse ramp/fall time control methods have active circuitry, which consumes power, and are costly due to the complicated circuitry and large footprint required. Another significant disadvantage of these methods is their poor performance at temperature extremes. The cavity bandpass filter is a large assembly external from the power amplifier and has undesirable high insertion loss which significantly reduces the total power output of the power amplifier.